1. Field of the Invention
This invention relates to a process for manufacturing a stiffened panel made of composite material.
2. Description of the Related Art
As illustrated in FIG. 1, a stiffened panel 10 comprises, on the one hand, a panel 12 that forms a skin and that consists of fibers that are woven in a resin matrix and, on the other hand, long-member reinforcements 14.
The invention relates more specifically to the long-member reinforcements that comprise a non-flat central part with caps on either side in contact with the panel once assembled, whereby said central part is separated from said panel to be reinforced. By way of example, this reinforcement can have an omega-shaped profile.
In the case of an aircraft, the fuselage comprises, on the one hand, a skin, and, on the other hand, a first series of stiffeners with Omega-shaped cross-sections, flattened against the inside surface of the skin and arranged in a longitudinal direction and also called stringers, and a second series of stiffeners arranged in planes that are perpendicular to the longitudinal direction and also called frames. Thus, the fuselage can come in the form of a stiffened panel or a set of juxtaposed stiffened panels connected to a structure that consists in particular of frames.
According to a first operating mode that is illustrated in FIG. 2, the stiffened panel is produced using a molding device 16 that is rigid in contact with the surface of the panel that comprises the stiffeners.
In this case, this rigid molding device 16 comprises grooves 18 whose profiles are matched to those of the stiffeners 20. Each groove 18 has a central part that corresponds to the central part of the stiffener with—on either side—hollow lateral housings 22 that are matched to the caps of the stiffener.
The depth of the lateral housings 22 is adapted to the thickness of the caps of the stiffener in such a way that the surface of the caps that will be flattened against the skin is essentially at the same level as the surface of the device 16 that will be in contact with the skin. Relative to the width of the lateral housings, there can be a play J between the end of each cap of the stiffener and the edge of the corresponding lateral housing 22.
After the installation of stiffeners, a core 24 is connected at the hollow shape of each stiffener 20.
After the installation of the cores 24, the deposition of fibers of the skin 26 is initiated. This operation can be performed using a draping machine or more generally a fiber-placing machine.
The core 24 is to be rigid enough to withstand compression forces generated by the machine during the deposition of fibers.
Prior to the polymerization, a flexible and sealed bladder 28, which comes into contact with the device 16 on the periphery of the panel, is connected to the unit. Drainage means are inserted between the skin 26 and the sealed bladder 28 to ensure the evacuation of gases during polymerization. Advantageously, a semi-rigid shaping plate 30 is inserted between the skin 26 and the bladder 28. Semi-rigid is defined as the shaping plate 30 being rigid enough to transmit compression forces to the skin and flexible enough to adapt to the radii of curvature of the skin.
Next, the unit is subjected to a polymerization cycle during which the walls of the stiffeners and the skin are compacted so as to reduce the porosity of said walls. Advantageously, each core 24 is placed in a bladder into which a pressurized gas is injected in such a way as to compact the walls of the stiffeners and the walls of the skin facing the core 24.
After the polymerization, the cores 24 are withdrawn.
This operating mode is not satisfactory on the following points:
When the play J is excessive, greater than 0.2 mm, during polymerization a pressure gradient develops perpendicular to this play J, which causes localized surface defects such as deviations of fibers. Furthermore, as illustrated in FIG. 2, perpendicular to the play J, the pressure exerted on the skin is not adequate so that the part can have too high a porosity.
According to another drawback, this pressure gradient generates a surface defect at the outside surface of the panel that has to be corrected by sealing.
According to another drawback, this operating mode requires the use of rigid cores, with dimensions calibrated to those of the recess of each stiffener, difficult to design and to use. Thus, with the stiffeners having different shapes, it is necessary to provide as many cores as shapes of stiffeners. According to another point, these cores are difficult to extract, in particular because of the length of the stiffeners, general curves of the stiffeners, or defects of local shape. In general, these cores are disposable and are dissolved for extracting them.
According to another drawback, a slight mismatch between the surface of the cap of the stiffener and the surface of the adjacent device can develop and produce localized surface defects at the skin.
Finally, according to another drawback, during polymerization, the skin is compacted between a rigid element, namely the device 16, and a semi-rigid element 30, which is sparingly compatible with a homogeneous compacting.
According to another operating mode illustrated in FIG. 3, the stiffened panel is produced using a rigid molding device 32 in contact with the surface of the panel that does not comprise stiffeners.
In a first step, the fibers that form the skin 34 are deposited on the device 32 whose surface in contact with said skin is shaped to the outside surface of the fuselage. This operation can be carried out using a draping machine or more generally using a fiber placement machine.
Next, the stiffeners 36 are arranged on the skin 34 by having taken care to insert a rigid core 38 between each stiffener and the skin.
Prior to the polymerization, a flexible and sealed bladder 40 that comes into contact with the device 32 on the periphery of the panel is connected to the unit. Drainage means are inserted below the flexible and sealed bladder 40 to ensure the evacuation of gases during the polymerization.
Next, a second “comb”-type device that makes it possible to maintain the relative positions of the stiffeners during polymerization is connected. This second holding device comprises, for each stiffener, two elements that are supported on either side of the stiffener on the bladder.
Polymerization is then initiated. The cores 38 should be rigid enough to maintain the shape of the stiffeners during polymerization. They are withdrawn after this phase.
As above, this operating mode is not satisfactory for the following reasons:
Under the effect of pressure and despite the flexibility, the bladder cannot homogeneously compact the zone that corresponds to the end of the caps of the stiffener, taking into account the local discontinuity.
As illustrated in FIG. 3, this zone is subjected to high pressure gradients that cause surface defects at the inside surface of the panel, and even the embedding of the end of the cap in the skin.
Furthermore, this operating mode requires the use of a rigid core with the same drawbacks as those mentioned for the first operating mode.
As above, the pressure gradients also generate surface defects at the outside surface of the panel.
According to another drawback, it is very difficult, and even impossible, to place the flexible bladder in such a way that it exactly follows the profile of a surface that cannot be developed and that comprises in addition stiffeners on a large surface that can reach more than 70 m2.
Finally, according to another drawback, the bladder can be used only for the manufacturing of a single panel and after its use constitutes scrap to be destroyed.